by Raúl Porcar García and Julián A. Restrepo R.*
Many of the solvents used today by the paint industry have been recognized for their flammability, dangerousness, toxicity and environmental impact. This has led to the legislation increasingly generating stricter standards for the sector, provoking the rapid response of the industry, not only seeking to reduce the impact caused on the environment of its manufacturing processes, but the reduction of the consequences inherent to its products and the search for alternatives.
Under this scheme, more environmentally friendly alternatives have emerged, such as green solvents, also called alternative solvents or new solvents, which are a series of substances that are considered as alternatives to the current ones to reduce the impact caused by the use of conventional organic solvents. That is, more environmentally friendly solvents for the reduction or elimination of the generation of VOCs (volatile organic compounds).
Considering the point of view of "Green Chemistry", "alternative solvents" are included in the so-called "neoteric solvents", and if in this classification we omit water because it is a solvent already used for several years by the paint industry, we have that among these new solvents are, mainly [1]:
- Renewable solvents: Obtained from renewable raw materials, also called "bio-solvents".
- Fluorinated Solvents: Which instead of being hydrocarbon solvents (HC) are fluorocarbon solvents (FC).
- Supercritical Fluids (FSCs): The most common uses are in the use of supercritical water (scH2O) and supercritical CO2 (scCO2), as a replacement for halogenated solvents, such as dichloromethane, in industrial processes.
- Ionic Liquids: Their biggest advantage as "green solvents" is their low volatility at room temperature, so they are not considered VOCs.
Ionic liquids: a green alternative
Ionic Liquids (LIs) are ionic compounds formed by the combination of a cation and an anion, such as salts, so the term "molten salts" is also used to refer generally to LIs.
It should be clarified that the difference between ionic liquid and molten salt is a bit artificial, using by convention the general term of LIs to designate those ionic compounds whose melting temperature is below 100ºC. Comparatively, a salt such as sodium chloride (common salt) has a melting point greater than 800ºC, being solid at room temperature. While for those ionic compounds that are liquids at room temperature (that is, with melting points below 25ºC), the term ionic liquids at room temperature or RTILs is used more specifically. In reality, LIs are not new substances, since the use of molten salts in different applications was already known since the nineteenth century, being the first LIs based on alkylamonic salts [2].
LIs have a number of advantages over many of the substances we use as solvents, such as excellent thermal and chemical stability, virtually zero volatility (due to their low vapour pressure), high thermal capacity and high resistance to combustion; low melting points, are liquids in a wide temperature range (between -50 and 250ºC) and have a high solvent power for many substances (their high solvent power allows them to be used in chemistry as a reaction medium). In addition, they are not corrosive or flammable, have viscosities similar to traditional organic solvents, as well as are recyclable and can even be used as catalysts.
LIs are also called "design solvents", since a large number of cations and anions can be used for their synthesis in order to obtain compounds with various properties. Thus there is a great variety of LIs, whose properties depend on the nature of the constituent ions and chemical structure, such as their miscibility in water and organic solvents: you have LIs soluble in water and others are completely insoluble in it (even if these are very hygroscopic).
The use of LIs is interesting due to the possibility of replacing traditional organic solvents, which will allow considerable environmental improvements, minimizing the emission of VOCs into the atmosphere. The strict non-volatile nature of LIs offers an opportunity to reduce, or even completely eliminate, the emission of toxic and hazardous substances into the atmosphere and thus achieve environmental benefits. That is why LIs have been gaining interest in many fields, including organic chemistry, electrochemistry, catalysis, physico-chemistry and engineering, even having applications of magnetic ionic liquids.
In general, what makes a solvent green is its use within a specific application, since none of the main properties of LIs alone gives rise to cleaner or less polluting processes, this is only achieved when an increase in efficiency, a decrease in waste and a minimization of material losses are achieved. In this way, if an LIs is able to meet these conditions, it can be classified as a green solvent and incorporated into the various applications of Green Chemistry.
In general, it can be noted that the numerous applications of LIs have significantly reduced the environmental impact compared to conventional solvents, improving the efficiency of the process and minimizing the waste generated. Some applications as solvents
LIs are considered one of the alternatives for the reduction or replacement of VOCs, because as mentioned, they have practically zero volatility.
In the commercial development of LIs as solvents, we have attractive examples such as that of the German company BASF, which offers about 20 LIs under the Basionics trademark, for other companies that wish to develop their own processes based on LIs, in fact, BASF goes further and wants to develop LIs available on a large scale at reasonable prices and offer technical support to its customers for the optimization of their processes based on LIs [3].
Another company that has developed commercial LIs is the North American n-Gimat, which offers LIs based on 2-ethylhexanoate salts of metals, such as cobalt (II), nickel (VI) and zinc (II), and others with cations of an organic nature, such as choline (a saturated quaternary amine) and ethylenediamine [4]. All of them liquids at room temperature (RTILs), except cobalt (II), and in addition, they are colorless, except nickel which is green in color. An additional environmental advantage is the fact that they are free of halogenated compounds.
Another example is in liquid phase reactions, where the use of phosphonium LIs compatible with strong bases is reported, which allows reducing the generation of VOCs and the use of halogenated solvents [5].
There are also interesting studies for the use of LIs as green solvents for compounds from renewable materials, such as cellulose [6] and lignin [7]. These studies are very important, bearing in mind that cellulose and lignin are, in their order, the most abundant biomaterials on earth, in addition to the fact that they are compounds present in most wood resources and that their derivatives have important applications in the fiber, paper, paints industry, in the manufacture of membranes and polymers, so they have an important potential in a framework of sustainable development.
But it must be said that despite the numerous studies for the use of LIs as solvents, due to their high cost and because they are products that are still in the development stage, they are not currently widely used as solvents and are only used in a few industrial processes.
The main processes that can be mentioned and that employ LIs are:
- The BASIL process (Biphasic Acid Scavenging utilising Ionic Liquids), developed by BASF for the production of alkoxyphenylphosphines [8], with which it received the ECN innovation award in 2004, for the development of the first large-scale process that used LIs in an industrial process.
- The Difasol process developed by the French Petroleum Institute (IFP), which is based on the dimerization of alkenes, usually propene and butene, to produce hexene and octet, using an ionic liquid as a solvent and a nickel catalyst [9].
- It should also be mentioned that the biphasic hydrosilation process in LIs is ready for industrial implementation in the coming years [10].
Due to their low volatility characteristics, in addition to solvents, LIs can be used as electrolytes, in devices for heat storage, in metal processing, separation and purification processes, as well as in paints they can be used as additives. Thus, this section will show that LIs are not only used as solvents but also have other applications of high industrial and environmental interest.
LIs, in addition to being more environmentally friendly, have some additional advantages that other solvents do not have, such as [4]:
Dissolution of metal oxides, minerals and refractory materials (they can be used in the production of metal oxides).
Many are immiscible with water, so they can be used in separation processes.
They dissolve hydrocarbons of both polar and non-polar nature, which makes them very versatile for use in organic synthesis.
Specifically in the area of paints and coatings, LIs can be used as "performance additives", where a greater and more interesting application potential for these is glimpsed, of which some examples can be mentioned:
- The German company Degussa uses LIs as additives in the manufacture of new paints (see figure), with the aim of improving the finish, appearance and drying properties, this allows the reduction of the use of VOCs [11]. Another application, which Degussa investigates, is in the use of LI in lithium batteries as electrolytes.
- Studies report that LIs can be used as plasticizers of polymethylmethacrylate (PMMA) and polyvinylchloride (PVC) [12, 13].
- There is also research in which LIs have been used as dispersants and surfactants to stabilize pigments, paints and lacquers, as well as can be used in the formulation of detergents [14].
- Additionally, there are studies of their use as biocides, having an inhibitory effect against different bacteria and fungi [15].
- Finally, it should be mentioned that in addition to presenting many other potential applications, LIs can also be biodegradable [16] or obtained from renewable sources [17], which makes them have a greater potential for application in the production of more environmentally friendly products.
[1] Restrepo, J. A. Inpralatina, Vol. 13, No. 4, July/August 2008, p.28-31.
[2] Gabriel, S.; Weiner, J. Chemische Berichte, 21 (2), (1888), p.2669–2679.
[3] Baker, J. Eur. Chem. News, Sept-Oct., 2004, p.18-20.
[4] Visit: www.ngimat.com
[5] Ramnial, T.; Ino, D.D. and Clyburne, J.A.C. Chem. Comm., 2005, p.325-327.
[6] Swatloski, R.P.; Spear, S.K.; Holbrey, J. D. and Rogers, R. D. J. Am. Chem. Soc., 2002, 124, p.4974-4975.
[7] Pu, Y.; Nan, J. and Ragauskas, A.J. Jour. Wood Chem. & Tech., 2007, 27, p.23-33.
[8] Visit: http://www.basf.com/corporate/051004_ionic.htm.
[9] Masse, M. Multiphase Homogeneous Catalysis, Ed.C. Boy, Wiley-VCH, Weinheim, 2005, p. 560.
[10] Geldbach, T.J.; Zhao, D.B.; Castillo, N.C.; Laurenczy, G.; Weyershausen, B.; Dyson, P. J. J. Am. Chem. Soc., 2006, 128, p.9773-9780.
[11] Weyershausen, B, and Lehmann, K. Green Chem., 2005, 7, p.15-19.
[12] Scott, M.P.; Brazel, C.S.; Benton, M.G.; Mays, J.W.; Holbreyc, J.D. and Rogers, R.D. Chem. Comm., 2002, p.1370–1371.
[13] Zhao, H. Chem. Eng. Comm., 2006, 193, pp.1660–1677.
[14] Baker, G. A.; Pandey, S.; Pandey, S. and Baker, S. N. Analyst, 2004, 129, p.890-892.
[15] Pernak, J.; Sobaszkiewicz, K. and Mirska, I. Green Chem., 2003, 5, p.52–56.
[16] Garcia, M. T.; Gathergood, N. and Scammells, P. J. Green. Chem., 2005, 7, p. 9–14.
[17] Imperato, G., König, B. and Chiappe, C. Eur. J. Org. Chem. 2007, p. 1049–1058.
[18] Visit www.invesa.com, products that are friends of quality and the environment.
[19] J.A.R. thanks the AlBan-Fundación Banco Santander Program for the financial support for the granting of the scholarship for postgraduate studies No. Q07D402098CO.
* Technology Center, INVESA S.A.(a)
Dept. of Inorganic and Organic Chemistry, Universitat Jaume I (b)
[email protected]
Castellón, Spain
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